[mips] [IAS] Add support for the .set softfloat/hardfloat directives.

Summary: These directives are used to set the current value of the SoftFloat feature.

Reviewers: dsanders

Reviewed By: dsanders

Subscribers: llvm-commits, mpf

Differential Revision: http://reviews.llvm.org/D9074

llvm-svn: 238813